This invention relates to a vehicle brake control system and, in particular, to a brake feel augmentation valve to be used in an aircraft skid control system.
Prior state-of-the art vehicle skid control systems are described in the inventor's prior U.S. Pat. Nos. 4,053,187, 4,130,322 and 4,260,198. In these systems the vehicle operator (e.g., an airplane pilot) initiates braking action by depressing a brake pedal. The brake pedal is connected to a pressure metering valve which supplies hydraulic fluid to the brakes via a skid control valve. A skid control circuit monitors the output of a wheelspeed transducer to check for skid conditions. If a skid is detected, the skid control circuit sends a control signal to the skid control valve. The skid control valve will regulate the amount of hydraulic fluid the metering valve supplies to the brakes, and thereby control braking action to prevent skidding.
While the brake control system disclosed in the foregoing patents has worked well in aircraft equipped with steel brakes, it has been found that the system could be improved when applied to new aircraft equipped with carbon brakes. Specifically, in the prior patented systems the brake cylinder is filled with hydraulic fluid directly by the pressure metering valve. When the pilot pushes down the brake pedal to initiate braking, the pedal will depress far because the brake cylinders are empty, and the pedal will feel "soft" during the time it takes the brakes to fill. A pressure drop will be created across the pressure metering valve while the brake cylinders fill. However, the brake cylinders fill with hydraulic fluid quickly, which sends an acute hydraulic force to the brake pedal via the pressure metering valve. In carbon brakes, the torque gain at low brake pressures is typically two to four times that of steel brakes. Thus, the acute hydraulic force regularly can be large enough to kick back at the pilot's foot through the pedal, which makes the brakes difficult to control.